Hi!
Just my own experience:
I have both the Tamron 1.4x Pz teleconverter and the A-FA 1.7X Pentax adapter.
Since I acquired the Pentax one, I have not used the Tamron a single time.... Oh! ... yes I did, once only with my Tamron 70-200/2.8.
The IQ of the 1.7X adapter, mounted with the DA*300/4 is very good, if not great. I took me quite a while to get used to go "manual focusing" and letting the adapter do its magic though. I admit not using it often enough, probably because I end up shooting in less than adequate lighting conditions very often. But I find that it is very fast and accurate with proper exposure.
Under good lighting, such as during a bright morning, the combo I have described above works without a glitch.
And, if I remember correctly, the 1.7X TC plus the DA*300/4 will give me a maximum aperture of 6.3 (or is it 6.7? ...
); anyway, this tells you right away that it isn't perfect for low light conditions, especially when one needs faster shutter speeds. ... repeating myself here.
Hope that helps.
JP